Ozone production efficiency in Savanna and forested areas during the EXPRESSO experiment

NO, NOx, NOy and O-3 have been measured during the airborne EXPRESSO experiment, November 96, which took place near Bangui (Central Africa) at the beginning of the dry season. This period corresponds to an intense burning activity. Chemical and photochemical characteristics of the planetary boundary layer, which corresponds most of the time to the monsoon layer, and the Harmattan layer, which is situated above the latter, have been studied for savanna as well as rain forest areas. These two layers are very different when considering the chemical composition (especially for ozone and NOz) and the photochemical age. The relationship of photochemical ozone production versus photochemical NOx oxidation products has been investigated. Results indicate an ozone production efficiency (OPE) ranging from 6.3 to 14.8 in the planetary boundary layer. Thus, this layer is characteristic of a photochemically young and active air mass. In this layer, the ozone potential production increases with the air mass photochemical age. On the other hand, the Harmattan layer shows an OPE ranging from 4.6 to 7.4. These values are characteristic of photochemically well-aged air masses. In this layer, the ozone potential production seems to be exhausted with values around 4.0 (i.e., 4 ozone molecules produced for each NOx molecule emitted).